1. Field of the Invention
This invention relates generally to the field of optical communications and in particular to a tunable spectral filter for selectively attenuating and/or switching—as a function of wavelength—the optical power of an optical communications signal.
2. Description of the Prior Art
Fourier-plane (FP) filters are one class of prior-art optical filter components. In FIG. 1, there is shown such a prior art filter 100. In such FP filters, light from an input fiber 101 is collimated by a lens 102, and aligned so that a collimated beam illuminates a permanent optical spectral filter 103 located in the collimated, or Fourier-transformed, plane. Light that is reflected by the permanent optical spectral filter 103 is focused by a second, reverse pass through collimator lens 102 into an output fiber 104 thereby forming a two-port optical filter. Similarly, a three-port filter may be constructed by aligning a second collimator lens 105, such that light which is transmitted by the filter 103 is directed into a second output fiber 106.
As can be readily appreciated, the optical spectral filter 103 might be a fixed dielectric stack, transmitting a pre-determined optical wavelength band or spectral profile. A dynamic two-port spectral filter replaces the fixed filter 103 with an electrically-controlled element to modify the spectral reflection and transmission profile, as for example the micromechanical etalon filter described in the article entitled, “Widely tunable Fabry-Perot filter using Ga(Al)As—AlOx deformable mirrors” authored by P. Tayebati et al, and which appeared in IEEE Photonics Technology Letters, pages 394-396, in March 1998. In this device described therein, a voltage adjusts an air space between two reflective layers to control the center wavelength of a narrow transmission band. A tunable spectral filter based on such a device is currently manufactured by Nortel Networks®. In one version of this commercially available component, model number MT-15-300, a voltage from 0 to 50 V controls the center wavelength of a 0.35 nm wide transmission notch within a 100 nm tuning range. Such a filter is used, for example, to select a single wavelength data transmission signal. Other versions of this component have narrower 0.06 nm and 0.02 nm transmission bandwidths (model numbers MT-15-100 and MT-15-25, respectively). Such filters are typically used in optical spectrum analyzers, which measure the average power as a function of wavelength in a WDM transmission system.
Tunable, Fourier-plane (TFP) filter devices are simple, compact and relatively inexpensive. However, the only degree of control afforded by TFP filter devices is in the center wavelength. The different tunable filter models described above are necessary because the etalon bandwidth is determined by the reflectivity of the etalon surfaces, which are not adjustable. Also, during tuning from one wavelength to another, the filter transmits a continuously shifting band of wavelengths until the new center wavelength is reached. This type of tuning is called “hitting”, as the filter must sequentially hit each wavelength between the initial and final tuning positions. Finally, the fundamentals of etalon filter design impose restrictions on the spectral filter profile and do not allow, for example, an ideal square passband with uniform low loss across the allowed wavelengths and high extinction of the out-of-band signals.
A more complex type of prior-art spectral filter, which may be called a switched spectral-plane (SSP) filter, is based on a free-space optical spectrometer where the multiple wavelength input signal is imaged through a diffraction grating onto a spectrally demultiplexed image plane so that the lateral position of the signal corresponds to the incident wavelength. Passive free-space wavelength demultiplexing optical systems are described, for example, in the textbook entitled Wavelength Division Multiplexing, authored by Jean-Pierre Laude and published by Prentice Hall in 1994 as part of their International Series in Optoelectronics. Simple active versions of such free-space optical spectrometers use single-axis rotation control of the grating, as used for example in tunable wavelength lasers (e.g., J. Berger et al, “Widely tunable external cavity diode laser using a MEMS electrostatic rotary actuator,” 27th European Conference on Optical Communication, Proceedings Vol. 2, pp. 198-199, 2001) but such control is limited to adjusting the center wavelength alignment of the system. In SSP filters, however, the passive spectrometer illuminates a linear array of independent optical actuators, which act to selectively switch or attenuate each spectral component of the multiple wavelength signals. A second pass through the free-space spectrometer optics can spatially recombine the switched or attenuated signals.
With reference now to FIG. 2, there is shown a specific example of such a prior art SSP filter 200 designed to provide arbitrary attenuation on each channel of a multi-wavelength signal for dynamic spectral power equalization. The basic concept of this SSP filter is disclosed in U.S. Pat. No. 5,745,271, for “Attenuation device for wavelength multiplexed optical fiber communications”, issued to Ford on Apr. 28, 1998, and the physical configuration shown is disclosed in U.S. Pat. No. 6,307,657 for an “Optomechanical platform”, which issued to Ford et al on Oct. 23, 2001.
With further reference now to FIG. 2, optical input signals are directed by input fiber 201 through an optical circulator 202 and enter a free-space optical system through input/output fiber 203. Light emitted from input/output fiber 203 is collimated by a first pass through lens 204 to illuminate a reflective diffraction grating 205. The diffraction angle is proportional to the wavelength, so grating 205 acts to separate each wavelength signal by angle. A second pass by the light through lens 204 to a spectrally dispersed plane 206 focuses the diffracted signals where each signal is vertically displaced according to the wavelength.
A micromechanical attenuation device 207, located at the spectrally dispersed plane, includes an array of individually controllable optical attenuators 208. Light which is reflected from the attenuator array retraces its input path as it is recollimated by a third pass through lens 204, diffracting again from grating 205, and finally focused back into the input/output fiber 203. For clarity, the arrows drawn in FIG. 2 indicate the first pass of the light through the optical system from input/output fiber 203 to attenuator array 208. On the return path from attenuator array 208 to input/output fiber 203 the direction of the light is reversed.
Attenuation device 207 may be designed so that each individual attenuator within optical attenuator array 208 absorbs a controlled portion of each wavelength signal, as described in the above-referenced U.S. Pat. No. 6,307,657. Other types of attenuation devices can also controllably reduce the amount of light that is coupled into the single mode output fiber 203.
Also shown in FIG. 2., is an attenuator device 207 that includes an attenuator array 208, having a linear array of tilting micro-mirrors, controlled by external electrical connections 209. When one individual micro-mirror of the attenuator array 208 is tilted, the corresponding wavelength signal imaged in a second pass through the optical system is incident on input/output fiber 203 at a controlled angle relative to the fiber face. The efficiency of coupling into the fiber depends on angle of incidence, so a controlled tilt of the micro-mirror controllable reduces the output power of the corresponding wavelength signal coupled back into the input/output fiber 203. Finally, the backwards-propagating output signals pass through the optical circulator 202 and are directed into a separate output fiber 210. Fiber optic components based on variations of this design are commercially available as, for examples, the Dynamic Channel Equalizer, marketed and sold by LightConnect, Inc., and the “AgileWave” (TM) Dynamic Spectral Equalizer, marketed and sold by Cidra, Inc.
As can be appreciated, SSP-type filters can provide wavelength switching as well as attenuation. An analog tilt mirror can be used to direct the demultiplexed signals into two or more distinct output paths, such that each wavelength signal is independently controlled. A wavelength add/drop switch which allows any combination of signals to be extracted from a multi-wavelength transmission and replaced with different data signals on the same wavelengths is described in U.S. Pat. No. 6,204,946, “Reconfigurable wavelength division multiplex add/drop device using micromirrors.” A related system which allows arbitrary 2×2 switching is described in U.S. Patent Publication No. US 2002/0009257 A1, entitled “Optical configuration for a dynamic gain equalizer and a configurable add/drop multiplexer” published Jan. 24, 2002. Finally, a system which allows arbitrary 1×4 switching is described by D. Marom in a conference paper presentation at the 2002 OSA/IEEE Optical Fiber Communications Conference, the presentation entitled, “Wavelength-selective 1×4 switch for 128 WDM channels at 50 GHz spacing,” which was published in the Postdeadline Proceedings pages FB7-1-3, in March 2002.
SSP filters offer a more versatile platform for spectral manipulation than simple FP filters, but this increased functionality comes at a cost in terms of optomechanical system packaging and control electronics. In particular, components that achieve switching or filtering at the individual wavelength channel level must maintain accurate alignment of the dispersed spectrum onto the corresponding switch elements to within a fraction of the channel pitch, which is typically 100 GHz pitch (0.8 nm) or even 50 GHz (0.4 nm). The minimum individual switch size of micromechanical and liquid crystal modulators is typically larger than the 10 micron mode field diameter of single mode optical fiber, making very fine wavelength pitches difficult to achieve without magnification. Also, a significant portion of the device area is dedicated to actuators (torsion bars or hinges) or close to the feature edges, and so is not optically usable, resulting in notches or gaps in the spectral response of the filter in the narrow bands between WDM communication channels. These gaps become problematic when multiple SSP filters, each with slightly different registration of the switching windows, are cascaded. Achieving a flat transmission passband over the channel transmission window, which is roughly half of the channel pitch, can force systems designers to use long focal length lenses (around 100 mm). Large optical systems that maintain micron-scale alignment over all temperatures and over the 20 year installed life of such components are expensive. Active temperature stabilization or feedback alignment systems are possible, but also incur manufacturing and operating costs. Therefore, SSP filters are highly functional but not inexpensive or compact.
A need therefore exists in the art for an inexpensive dynamic spectral filter that provides greater functionality than a FP tunable filter without the complexity and cost of a full SSP filter.